1. Field of the Invention
The present invention relates to an impact sensor mounting structure for an automotive vehicle and, more particularly, to a structure for mounting an impact sensor at a front portion of a vehicle body. The mounting structure is especially useful in a vehicle having a relatively short nose, and permits the impact sensor to sense a crash of the automotive vehicle with a desired degree of responsiveness.
2. Description of Related Art
In recent years, there has been a growing tendency to load an air bag system in an automotive vehicle as a security measure. Such an air bag system is arranged so as to absorb crash energy acting upon a driver or one or more passengers and prevent the driver, passenger or passengers from crashing into a steering wheel or front window. The air bag is expanded outward from a steering wheel or an instrument panel upon a crash of the automotive vehicle. The crash of the automotive vehicle is detected by a plurality of impact sensors mounted to the vehicle body. (See Japanese Laid-Open Utility Model Application (Kokai) No. 2(1990)-100,962).
In conventional air bag systems, it is common to use a disposable solid type of gas inflater, and to operate the gas inflater only when needed so as to decrease vehicle maintenance costs.
Typically, an impact sensor system includes a plurality of front sensors mounted to a front end portion of front frames and an approximately center portion of a shroud upper member, and in a vehicle compartment at, for example, a downward end portion of the instrument panel. This type of sensor system is generally arranged so that the compartment sensors are highly sensitive while the front sensors are less sensitive. Further, the air bag system is conventionally provided with an AND circuit arranged so as to accept two signals indicative of a car crash from at least one of the front sensors and at least one of the compartment sensors. If two such signals are accepted, a signal for expanding the air bag is generated. The sensitivity and the mounting structure of the impact sensors should be arranged so as to prevent the air bag from expanding upon a light crash which does not adversely affect the driver, the passenger or the passengers. Such a light crash, for example, may be a crash which occurs at a vehicle speed of less than 8 miles/hour. The sensitivity and mounting structure of the impact sensors, however, should also be arranged so as to expand the air bag upon a middle sized or heavy crash, since such a crash may possibly adversely affect the driver, the passenger or the passengers. Such a middle-sized or heavy crash may, for example, be a crash which occurs at a vehicle speed of more than 16 miles/hour. Conventionally, an air bag system is arranged so that those impact sensors which are mounted at approximately the center portion of the shroud upper member are located in front of or over the shroud upper member so as to prevent the crash energy from directly passing through the impact sensors. The crash energy is, for example, at least partly absorbed by a bumper. An impact sensor mounted at the shroud upper member then generates a signal indicative of the car crash only when the crash energy is higher than a predetermined level.
An impact sensor mounted to the center of a shroud upper member is arranged so that it generates a signal indicative of a crash earlier than the other front sensors when the automobile crashes into an object such as a pole or which has a portion higher than the front frames of the vehicle and projects toward the vehicle. The vehicle may, for example, collide with a utility pole at the center of its front f ace or with a rear end portion of a high-deck type vehicle. In this situation, it is possible that the crash will not cause the other front sensors to properly generate their respective signals, because these types of crashes tend not to transmit the crash energy into the front frame to which the other front sensors are mounted. As a result, it is clear that the sensor mounted to the center of the shroud upper member should be located above the front frames.
In recent years, to satisfy automobile design requirements, a relatively short nose type automobile has been widely adopted. The relatively short nose type automobile is designed so that a distance between a front edge of the automotive vehicle and a front edge of a front window, the length of an engine compartment in front of a passenger compartment in the longitudinal direction, and the distance between a bumper and a shroud upper member are all shortened.
An impact sensor mounted to the shroud upper member is either located in front of or over the shroud upper member. During a car crash, before the bumper absorbs crash energy sufficiently, it is possible that a part of the bumper will touch the impact sensor so that the impact sensor will generate a signal indicative of the car crash even though only a light crash has occurred.